Modern fire detection systems generally include smoke detecting elements. Prior art smoke detecting elements included ionization chambers containing a radioactive source, apparatus for measuring the attenuation of light, and apparatus for measuring resistance changes in thin film semiconductors. Light attenuation type smoke detectors are relatively complicated and, therefore, somewhat more expensive than thin film or ionization type detectors. The long term stability of thin film detectors remains unproven while the inclusion of radioactive sources may adversely affect the marketability of ionization chamber-type detectors.
Surface ionization on a hot filament in the presence of alkali metals has, in the prior art, been utilized for detection of halogen gases. Current flow between a heated filament and an adjacent cathode has been found to increase in the presence of a halogen-containing gas.